1. Field of the Invention
The present invention relates to a transfer apparatus, etc., for transferring a data stream such as an MPEG-2 transport stream.
2. Description of Related Art
Presently, an MPEG-2 transport stream (hereinafter referred to as the “MPEG2TS”) is known as an example of a data stream, as defined in standard ISO/IEC 13818-1: Information technology—Generic coding of moving pictures and associated audio information: Systems. The MPEG2TS is comprised of transport stream (TS) packets of 188 bytes in length.
Furthermore, a transfer apparatus configured to transfer an MPEG2TS using the IEEE 1394 standard is also known at present, as disclosed, for example, in Japanese Laid-Open Patent Application No. 2001-282713.
Moreover, using the IEC 61883 standard when transferring an MPEG2TS using the IEEE 1394 standard is also known, as defined, for example, in standard IEC 61883-1, Consumer audio/video equipment—Digital interface—Part 1: General, and standard IEC 61883-4, Consumer audio/video equipment—Digital interface—Part 4: MPEG2-TS data transmission. In the standard IEC 61883-1, Consumer audio/video equipment—Digital interface—Part 1: General, the data format of a common isochronous packet (CIP) header is defined. In the standard IEC 61883-4, Consumer audio/video equipment—Digital interface—Part 4: MPEG2-TS data transmission, the data format of a source packet header (SPH) is defined. FIG. 1 shows the data format of the CIP header. FIG. 2 shows the data format of the SPH. In FIG. 2, a CYCLE_COUNT field 201 (13 bits) serves as an 8-kHz counter. A CYCLE_OFFSET field 202 stores an offset value. The SPH is used to adjust the timing for inputting each TS packet to an MPEG2TS codec (coder decoder).
In a communication system using both the IEEE 1394 standard and the IEC 61883 standard, transferring TS packets is controlled according to the SPH and a program clock reference (PCR), as shown in lines A to C in FIG. 3.
In the line A in FIG. 3, TS packets 301 and 302 each are a TS packet having a PCR. The PCR is information for adjusting a time interval. As shown in FIG. 4, the PCR has a Program_clock_reference_base field 401 (33 bits) serving as a 90-kHz counter and a Program_clock_reference_extension field 402 (6 bits) serving as a 27-MHz counter. The time interval between the TS packets 301 and 302 corresponds to a difference in value between a PCR included in the TS packet 301 and a PCR included in the TS packet 302.
In the line B in FIG. 3, the TS packet 301 is transferred onto an IEEE 1394 serial bus at the timing indicated by a TS packet 303. The TS packet 302 is transferred onto the IEEE 1394 serial bus at the timing indicated by a TS packet 304. In this instance, the time interval between the TS packets 303 and 304 is adjusted and changed to an integral multiple of 125 μs (microsecond). In this case, an SPH that is generated according to the PCR value is added to each TS packet.
In the line C in FIG. 3, a transfer apparatus on the receiving end adjusts the time interval between the TS packets 305 and 306 by the use of the SPH to become equal to the time interval between the TS packets 301 and 302.
However, the above-described configuration necessitates a large-sized buffer for eliminating a time lag by the use of the SPH (buffer memory for timing adjustment). This increases production cost of the transfer apparatus on the receiving end.